The regularity of the boundaries are typical of a sanding operation on wood that is not uniformly hard , so Hank`s question on whether there was a pre-coat of anything oily or waxy that was subsequently sanded off is pertinent . Which also leads to the question of the type of sandpaper you used . Some are stearate coated . I don`t use them , but it may be remotely possible that it might interfere with CA adhesion .

Last question would be , has the affected area grown since you first noticed it .